This invention relates to an instantaneous adjustment device for a doctoring blade assembly operatively linked to a cylinder in a printing press.
It is a known fact that the composition rollers of printing presses, and especially rotary presses, for example, incorporate doctoring blade assemblies which have, on their side confronting the rotating cylinder, a doctoring blade comprising a thin and highly flexible blade whose end is made to rest on the skirt surface of the rotating cylinder. Said doctoring blade is carried on an oscillating assembly which has, on its remote side from the doctoring blade, a rigid overhanging arm to which the end of a rod forming a part of a cylinder-piston assembly is connected. By supplying said cylinder with compressed air, for example, at a preset pressure level, it can be arranged that the knife edge of the blade bears on the cylinder at all times with a desired operating pressure. That pressure should be neither too low nor too high, if the doctoring blade is to constantly exert a desired pressure on the cylinder, an excessively high pressure being, however, to be avoided not to damage the cylinder.
It has now been found that, with such adjustment units linked operatively to the doctoring blade, after a few thousandths of a millimeter have been lost by the knife edge of the doctoring blade to wear, the gap between the rotating cylinder and doctoring blade increases undesirably, since the frictional forces developed, for instance, between the air-operated cylinder and respective piston are of such magnitudes as to inhibit an immediate compensation for the increase, within the range of a few thousandths of a millimeter, in the gap between the skirt of the rotating cylinder and the doctoring blade knife edge. It is necessary for the gap to become wider such that the slit may grow to a suitable size for the inertia of the adjustment cylinder-piston to be overcome and the cylinder-piston advanced by a very small amount to again press the doctoring blade knife edge against the skirt of the rotating cylinder. This inertia of the adjustment or bias system which acts on the doctoring blade assembly is the more significant the denser is the ink being used.
An increased gap between the rotating cylinder and knife edge of the doctoring blade, beyond a given size, however, brings about an undesired ink residue formation on the rotating cylinder, which results in haze formations on the printed product constituting a serious print defect yielding an inferior quality product.